The present invention relates to processes for producing foams using a physical blowing agent such as carbon dioxide, water and/or nitrogen.
Styrenic and olefinic polymers have been utilized in food packaging applications such as non-foamed thermoformable sheet used in dairy containers and the like. This type of thermoformable sheet has been typically produced using a conventional sheet extrusion line, wherein the polymer is extruded from a flat sheet die onto a three-roll polishing stack and either wound or fed directly into a thermoforming process. However, ecological influences are driving the food packaging industry to produce lighter packaging materials, due to the fees based on packaging weight which are applied to packaged food processors.
One way to reduce the weight of a thermoformable sheet is to incorporate a blowing agent to produce a high density foam sheet. Foam sheet of this type is typically produced using a conventional sheet extrusion line and a chemical blowing agent. However, chemical blowing agents are expensive and do not provide consistent or uniform foaming, resulting in foam gauge and density variation within the foam sheet produced.
Therefore, there remains a need for a process of producing high density foamed packaging materials, particularly thermoformable foam sheet having consistent gauge and density, which can be used in traditional non-foam applications such as dairy containers.
The present invention is a process for producing a thermoplastic foam sheet comprising contacting a molten monovinylidene aromatic or olefinic polymer with a physical blowing agent to form a polymer/blowing agent mixture, foaming the mixture into a region of lower pressure, extruding the foaming mixture to form a foam sheet, pulling and compressing the sheet to form a foam sheet having a uniform thickness.
The present invention also includes the thermoplastic foam sheet made by the process described above, as well as thermoformed articles produced therefrom.
The process of the present invention is especially suited for producing high density foamed packaging materials, particularly thermoformable foam sheet having consistent gauge and density, which can be used in traditional non-foam thermoformed applications such as dairy containers.